1. Field
The presently disclosed subject matter relates to a surface light source device and a LCD unit including the same, and more particularly to a surface light source device which can be configured to exhibit high brightness and uniform brightness and can utilize a plurality of line light sources. Thus, the surface light source device can be employed as a light source for a back light unit located adjacent a liquid crystal display (LCD) of a television and the like. Therefore, a LCD unit including the same can exhibit high brightness and uniform brightness even on a large screen.
2. Description of the Related Art
Conventional optical transmission displays such as LCDs and the like are typically illuminated from a rear portion thereof by a surface light source device in order to light their displays. In a color LCD unit used for a personal computer or a television, the light transmission rate thereof decreases due to various reasons including a restriction of an aperture ratio, a color filter, etc. Thus, the LCD unit is illuminated from its back using a back light unit like the surface light source device.
The surface light source device for a back light unit that illuminates from the back of the display unit can be broadly classified into an edge light type and a direct light type (when classified by location of the light source). The edge light type is composed of a light guide and at least one light source, which is located at least one edge of the light guide so as to emit light from one surface of the light guide. Thus, in an edge light type, the light-emitting direction of the surface light source device is substantially perpendicular to that of the at least one light source.
On the other hand, the surface light source device of the direct light type is composed of a diffusing member and a plurality of light sources, which are located rearward of the diffusing member so as to emit a diffusing light from the plurality of light sources via the diffusing member. Thus, according to the direct light type device, the light-emitting direction of the surface light source device is basically or substantially in the same direction as the light emitting direction of the plurality of light sources.
Consequently, because the number of light sources in an edge light type device is limited due to their location about a perimeter of the light guide, the surface light source device of the edge light type is sometimes not suitable for use as a back light unit when the device is of a large size and/or requires high brightness. However, because the direct light type device can include many light sources, and light emitted from the light sources can be directly emitted in a direction parallel with or towards the light-emission direction of the general surface light source device, the direct light type device is often employed for the back light unit when requiring a large size and high brightness, for instance, in a LCD television monitor of a large size, a LCD monitor of a computer, etc.
In the surface light source device of the above-described direct light type, a plurality of point light sources such as LEDs and the like are often used to constitute the light-emitting device. For example, in a method for making a surface light source device that emits white light, multicolor LEDs can be used in accordance with principles of additive color mixture. Specifically, the surface light source device can emit white light by mixing three different colored lights including a red LED, a green LED and a blue LED via a light-diffusing member. A method for making and using a white LED can also be employed in the surface light device and can include coating a yellow phosphor on a blue LED chip.
However, the direct light type surface light source device includes a problem in that a uniformity of the brightness thereof decreases when making a large size device and/or high brightness device. On the other hand, when trying to make a surface light source device having uniform brightness, the brightness of the surface light source device decreases and/or a thickness thereof becomes thick. Therefore, various structures used in surface light source devices that are large in size and/or emit a uniform brightness have been devised.
For example, a method for improving the uniformity of the brightness of a LCD panel is disclosed in Patent Document No. 1 (Japanese Patent Application Laid Open No. JP2006-058481). According to a back light unit using the surface light source device disclosed in Patent Document No. 1, the back light unit is composed of a light source unit including many LEDs, and a light-diffusing plate. The light source unit including the many LEDs located at a predetermined interval is located rearward of a LCD panel. The light-diffusing plate is located between the LCD panel and the light source unit and provides reflecting patterns on one surface thereof facing towards the LEDs. Each of the reflecting patterns is located opposite the LEDs and is composed of a light-reflective ink based upon an ellipse, which has a minor axis that is longer than a diameter of each of the LEDs.
According to the back light unit using the surface light source device having the above-described structure, light emitted at a large angle from each of the LEDs enters the light-diffusing plate at an area other than the reflecting patterns located on the light-diffusing plate, and is emitted from the opposite surface of the light-diffusing plate while being diffused as it passes through the light-diffusing plate.
At the same time, light emitted at a narrow angle from each of the LEDs is reflected from/on the reflecting patterns of the light-diffusing plate and reflects again on a surface of the light source unit. Then, a part of the reflected light enters the light-diffusing plate from an area other than the reflecting pattern area of the light-diffusing plate and is emitted from the other opposite surface while being diffused during transmission in the light-diffusing plate.
Therefore, because bright light emitted at a narrow angle from each of the LEDs reflects on the surface of the light source unit and does not pass through the reflecting patterns of the light-diffusing plate, the various beams or rays of light emitted from each of the LEDs are mixed with each other and become an approximately uniform brightness as a whole. Thus, the back light unit using the above-described structure can prevent irregular brightness and irregular color because the light emitted from the other/opposite surface of the light-diffusing plate becomes uniformly bright and has a uniform color by being diffused in the light-diffusing plate.
A back light unit using a surface light source device emitting a uniform white light is disclosed in Patent Document No. 2 (Japanese Patent Application Laid Open No. JP2005-339822). According to Patent Document No. 2, the back light unit includes: a reflector including a light-reflecting surface thereon; a plurality of light guides located on the light-reflecting surface of the reflector, and each of the plurality of light guides being composed of a transparent cylindrical body; and side-emission type LEDs including a red LED, a green LED and a blue LED located in each of the plurality of light guides, and each of the side-emission type three color LEDs including a total reflecting surface on a top surface lens thereof.
According to the above-described back light unit, because light emitted towards the top surface from each of the side-emission type three color LEDs reflects on the total reflecting surface located on the top surface of each lens of the LEDs, all light emitted from each of the side-emission type three color LEDs is substantially diffused in side directions. Therefore, because light emitted towards each top surface of the side-emission type three color LEDs decreases and light emitted in side directions increases, their light-pass length becomes long.
Consequently, the light rays emitted from each of the side-emission type three color LEDs are completely mixed with each other to achieve an approximately uniform brightness. Thus, the back light unit using the above-described structure can prevent irregular brightness and/or irregular color from occurring in a direction towards the light-emission thereof by maintaining a uniform brightness and a uniform color on an emitting surface of the above-described plurality of light guides.
On the other hand, Patent Document No. 3 (Japanese Patent Application No.
JP2006-304308) discloses a surface light source device that can effectively prevent irregular brightness and/or irregular color. FIG. 10 is a schematic cross-section view of the conventional surface light source device according to Patent Document No. 3 and FIG. 11 is a partial enlarged cross-section view depicting a relevant part of the conventional surface light source device shown in FIG. 10.
In FIGS. 10 and 11, the surface light source device 1 is composed of a circuit board 2, a plurality of LEDs 3, a light guide 4, a diffusing plate 5, a prismatic sheet 6, a diffusing reflection film 7 and a casing 8. The plurality of LEDs 3 include a first set of LEDs that are located at a predetermined interval in one direction on the circuit board 2 and formed flat, and another set of LEDs that are located at another predetermined interval in another direction perpendicular to the one direction. That is to say, the plurality of LEDs 3 is located on the circuit board 2 in a matrix state. In this case, for example, the plurality of LEDs 3 includes three different color LEDs including a red LED 3R, a green LED 3G and a blue LED 3B, and are dispersedly disposed so that the same color LEDs are not located adjacent to each other, as shown in FIG. 10.
The light guide 4 is located above the plurality of LEDs 3, and the diffusing plate 5 is located above the light guide 4. In addition, the prismatic sheet 6 is located above the diffusing plate 5. The diffusing reflection film 7 is located on the same surface of the circuit board 2 as are the plurality of LEDs 3 (with the exception of the mounting portion of the plurality of LEDs 3). The casing 8 holds the circuit board 2 and the light guide 4 so as to be able to illuminate a light emitted from the plurality of LEDs 3 via the light guide 4, the diffusing plate 5 and the prismatic sheet 6.
The light guide 4 is configured with a transparent material and a top surface thereof includes a plurality of curved surfaces 4a formed with a continuous arch in the one direction of the plurality of LEDs 3, as shown in FIG. 11. Each of base lines 4b located at the lowest portions on the plurality of curved surfaces 4a corresponds with each optical axis of the plurality of LEDs 3. In other words, the optical axis of the LEDs intersect with the base lines. Thus, a pitch of the plurality of curved surfaces 4a is the same interval as that of the plurality of LEDs 3 in the one direction and the plurality of curved surfaces 4a is formed convexly upwards between the respective adjacent two LEDs of the plurality of LEDs 3. The plurality of curved surfaces 4a are configured to reflect light entering into the light guide 4 on an inner surface thereof.
The light guide 4 includes a plurality of concave channels 4c formed concavely upwards and into a bottom surface thereof. The term concave is used to describe the inner surface of the channels 4c as viewed in the light emitting direction away from the device. The cavities or open portions of the channels 4c are actually concave when viewed in a direction from behind the device and in the light emitting direction. Each of the plurality of concave channels 4c is located in each line corresponding to each other direction of the plurality of LEDs 3. Each width of the plurality of concave channels 4c is wider than a width of each of the plurality of LEDs 3 so as to be able to dispose each of the plurality of LEDs 3 in each of the plurality of concave channels 4c. In addition, the light guide 4 includes a plurality of concave voids 4d between the respective adjacent two concave channels of the plurality of concave channels 4c on the bottom surface thereof.
The above-described top surface and bottom surface of the light guide 4 extend in accordance with the other direction of the plurality of LEDs 3. FIG. 12 is a schematic perspective assembly view depicting an attachment method for attaching the light guide 4 to the circuit board 2 in the conventional surface light source device 1 shown in FIG. 10. The casing 8 is configured with a box shape, which can include the circuit board 2 on a bottom surface thereof and can include an opening on a top surface thereof. The light guide 4 is attached to the casing 8 via the opening of the casing 8.
According to the surface light source device 1 of the above-described structure, a light emitted from each of the plurality of LEDs 3 enters into the light guide 4 from each of the plurality of concave channels 4c in which each of the plurality of LEDs 3 is disposed, and the light then reaches the respective inner surfaces of the plurality of curved surfaces 4a. In this case, because the plurality of curved surfaces 4a are configured to reflect totally the light on the respective inner surfaces thereof regardless of incidence angle, the light emitted from each of the plurality of LEDs 3 travels in an approximately sideways directions due to the total reflection on the respective inner surfaces as shown by arrow lines in FIG. 10.
One of the light rays reflected on one of the respective inner surfaces enters into the plurality of concave voids 4d on the bottom surface of the light guide 4 and reflexes/diffuses thereon. The other passes through the plurality of concave voids 4d and reflexes/diffuses on the diffusing film 7, and enters into the light guide 4 from the bottom surface of the light guide 4 again. Therefore, their lights entered into the light guide 4 are emitted outside from the top surface of the light guide 4 in a diffusing state and is also further diffused in the diffusing plate 5. Thus, the light emitted from each of the plurality of LEDs 3 is emitted from the surface light source device 1 in accordance with a predetermined controlling direction of the prismatic sheet 6.
According to the above-described structure, because the light emitted from each of the plurality of LEDs 3 is mixed with each other, the surface light source device 1 can emit an illuminating light having a relatively uniform brightness distribution on the whole. In this case, because light of different color is emitted from each of the plurality of LEDs 3 and then respectively mixed, the surface light source device 1 can emit illuminating light having a relatively uniform color distribution on the whole.
The above-referenced Patent Documents are listed below and are incorporated herein by reference.
1. Patent Document No. 1: Japanese Patent Application Laid Open JP2006-058481
2. Patent Document No. 2: Japanese Patent Application Laid Open JP2005-339822
3. Patent Document No. 3: Japanese Patent Application JP2006-304308
However, in the back light unit and surface light source structure disclosed in Patent Document No. 1, the reflecting patterns of the light-diffusing plate may be adjusted to have a diffusing rate corresponding to each optical axis of the LEDs at corresponding portions. In addition, because directivity characteristics and brightness of the LEDs is respectively different with respect to each other and with respect to each of LED products, the reflecting patterns may be required to be adjusted in terms of a density and distribution thereof with respect to each of the LEDs and each of the LED products in order to optimize a diffusing rate.
Thus, because the above-described adjustment is tough work, the back light unit may result in increased cost for the product. In addition, because the above described back light unit includes a process for manufacturing reflecting patterns, which is not required in a surface light source device of the conventional direct light type, and also includes a process for manufacturing the respective different reflecting patterns in each of the LED products, manufacturing costs for the back light unit may increase.
In the back light unit that includes the surface light source structure of Patent Document No. 2, because the light emitted from each of the LEDs is only transmitted in side directions using a directivity characteristic of the side-emission type LED, the reflector may be required to optimize a diffusing rate distribution on its light-reflecting surface. In addition, even when using the side-emission type LEDs, a part of the light emitted from each of the side-emission type LEDs that is not reflected on the top surface of the lens may leak out along a right overhead central axis near the top surface of the lens of the LED.
Thus, the back light unit may easily cause an irregular color, an irregular brightness, etc. Moreover, because each of the side-emission type LEDs may require a complex structure and a complex lens shape, the cost of the back light unit that uses many side-emission type LEDs may increase along with many man-hours.
In the surface light source device 1 shown in Patent Document No. 3, a point light source such as a plurality of LEDs 3 is used as the light source. The plurality of LEDs 3 is located in a matrix state. An irregular brightness may improve based upon the plurality of curved surfaces 4a formed on the light guide 4 in the one/first direction of the plurality of LEDs 3. However, because the plurality of LEDs 3 are disposed at an equal interval in the other direction perpendicular to the one/first direction, a brightness of positions between two respective adjacent LEDs with respect to each other may decrease. Thus, the uniformity of brightness of the surface light source device 1 may decrease.
In addition, because an area near the intersection of two diagonal lines of the plurality of LEDs 3 (as shown in FIG. 12) is far from each of the plurality of LEDs 3, a brightness of the intersection area may decrease. Thus, the surface light source device 1 may emit an irregular and/or decreased brightness. This is especially possible when high power LEDs are used for the plurality of LEDs in order to make the surface light source device 1 with a high brightness, and thus irregular brightness may be easily caused.
The disclosed subject matter has been devised to consider the above and other problems and characteristics. Thus, embodiments of the disclosed subject matter can include a surface light source device that does not cause (or depreciates) some or all of the above-described and other problems and that can emit a light from an arbitrary large area with high brightness and uniform brightness. In addition, the surface light source device can become thin even when increasing the brightness thereof and when the emitting area thereof becomes large. The disclosed subject matter can also include a LCD unit using the surface light source device as described. The LCD can be configured to increase the brightness of the display and provide uniform brightness, and to display at an arbitrary size and at a wide range, etc.